Astron. Astrophys. 336, 613-625 (1998)

1. Introduction

Resonance lines of neutral and singly ionized metals play an important role in the semi-empirical modelling of stellar spectra. Because they are strong and heavily dampened they may be used as probes of the plasma condition over a wide range of depths in the atmosphere from the outer most layers where the line core forms to the continuum forming region where the far wings form. In general, the use of the cores of these lines as plasma diagnostics is complicated by non-LTE effects that necessitate the solution of the coupled radiative transfer and statistical equilibrium equations. One of the ways in which a non-LTE solution may be especially complicated is the dependence of the equilibrium level populations on the non-local radiation field and, hence, on the detailed distribution of radiative opacity throughout the stellar atmosphere. This latter complication is especially problematic in M stars due to the massive over-blanketing of the radiation field by spectral lines from the far UV through to the yellow and red spectral regions.

The study of the outer atmospheric layers of cool stars is especially interesting because these layers exhibit chromospheric and transition region (TR) heating that is not yet fully understood. One way in which we can make progress is to more accurately measure the atmospheric conditions by developing new spectral diagnostics of the outer atmosphere and including them in multi-line model fitting. The value of this approach has been recently underscored by the discovery of an apparent discrepancy between newly developed diagnostics such as the CO band and traditional diagnostics such as the Ca II HK lines (Ayres 1990).

Andretta et al. (1997, ADB henceforth) investigated the non-LTE behavior of the Na I D lines in a grid of chromospheric/TR models that corresponds to an early dM star and spans a wide range of chromospheric activity levels. They found that the predicted line core responds to increasing chromospheric pressure in a way that is qualitatively similar to that of the well established diagnostics Ca II HK and Mg II hk. As the pressure increases from that of a quiescent, basal chromosphere to that of an active chromosphere, the absorption core first brightens, and then develops a central emission reversal. For those models in which the chromospheric temperature rise is shallower, the line core of the most active models exhibits a double reversal with a central absorption. As a result, ADB concluded that the Na I D line in early M dwarfs should be a powerful observational discriminator of chromospheric structure. ADB also investigated the effect of including photospheric line blanketing opacity in the calculation of the non-LTE H I and Na I D spectra. They found that the inclusion of background line opacity is especially important for the correct prediction of the line wing flux due to the suppression of the adjacent continuum.

We have obtained high resolution, moderate spectra of a sample of five early dM stars that span a range in chromospheric activity from quiescent to very active. We have recomputed the non-LTE H I and Na I spectra with a treatment of the background line opacity that is more complete than that of ADB (Short & Doyle 1997) and have performed a multi-line fit of a model chromosphere/TR structure to the observed H and Na I D line core for each of the program stars. In Sect.  2we describe the observations and data reductions; in Sect.  3we describe the modelling; in Sect. 4we present the results of the multi-line fit and compare them to the results of previous studies, and in Sect. 5we present our conclusions.

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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